Random Bits

searchfgold6789

I am having trouble understanding the RANDOM command in Basic Stamp Programming.

I am trying to simply generate a random number from 0 to 1, store it in a variable, and do it again in the same variable.

Is this possible?

Mike Green

The RANDOM statement takes a 16 bit integer and transforms it into another 16 bit integer in a way that has some of the properties of a sequence of random numbers, but is really deterministic. In your case, you want a random value between 0 and 1 which is not really doable given that all arithmetic on a Stamp is done on 16 bit integers, so there is no such thing as a floating point or fixed point value (other than integers).

When people need fractional values on an integer-only device, they usually use scaled arithmetic where they use integers to represent values with decimal points. For example, you can use 1534 to represent the fixed point value 15.34. The Stamp doesn't keep track of the number of decimal places. You have to do that and you have to explicitly provide for scaling when you multiply or divide these values together or change the number of decimal places. You also have to be careful of overflows since all arithmetic (including intermediate results) is done using 16 bit values.

What do you want to do with your "random" number?

searchfgold6789

Well I want to take my number which is either 0 or 1 (high or low), and put it in a variable, which is read and output PINS are turned on or off depending on whether or not the value is 1 or 0. Then the process is repeated 50000 times in a FOR...NEXT block, twenty times within another FOR...NEXT block. I will also DEBUG different strings depending on the result of the number.

Mike Green

OK, so you want an integer value of 0 or 1 that's randomly (or pseudo-randomly) distributed.

You do that by using the RANDOM statement on a variable, say Rand, and then using (Rand >>15) for your value.

Rand will be in the range 0-32767 half the time and in the range 32768-65535 half the time. (Rand >> 15) is a 0 for the range 0-32767 and a 1 for 32768-65535.

Tracy Allen

Another way to obtain psuedorandom bits is with a LFSR: linear feedback shift register. This does not use the RANDOM command. I'm still not sure if you want a whole variable (byte or word) to light a bank of LEDs, or one bit at a time.

' {$STAMP BS2pe}
' {$PBASIC 2.5}
randy VAR WORD   ' 16 bits for the shift register
newbit0 VAR randy.BIT0  ' its bit0 will be the random bit

main:
  randy = $5555 ' seed the sequence
  DO
    GOSUB taps
    DEBUG BIN1 newbit0
    PAUSE 16  ' do other stuff with the bit
  LOOP
  END

taps:   ' lfsr random bit generator
  randy=(randy<<1)+(randy.BIT15^randy.BIT14^randy.BIT12^randy.BIT3)
RETURN

searchfgold6789

Thank you! the program is working the way I want it to now.

searchfgold6789

Well now I am getting a confusing error... When I try to run
'{$STAMP BS2}
heads VAR WORD
tails VAR WORD
forum VAR WORD
'recount VAR BYTE
flip VAR WORD
heads = 0
tails = 0
flip = 0

'FOR recount = 1 TO 20

FOR forum = 1 TO 50000
DEBUG HOME
RANDOM flip
IF (flip >> 15) < 32767 THEN
DEBUG "Heads"
LOW 3
HIGH 2
heads = (heads + 1)
ELSE
DEBUG "Tails"
LOW 2
HIGH 3
tails = (tails + 1)
END IF
NEXT
DEBUG CR, ? heads, CR
DEBUG ? tails
'NEXT

LOW 2
LOW 3
END

I get

Error: Tokenization of /home/richie/Projects/BStamp/1000000CoinToss.bs2 failed.
Failed compile: 148-Expected a label
The following text gave the error:

What does that mean?

Mike Green

There are two versions of Parallax Basic, 2.0 and 2.5. The IF statement particularly is very restricted in version 2.0 and ELSE and ENDIF are not allowed. Since version 2.0 is the default for the Stamp Editor, you need to add a "' {$PBASIC 2.5}" directive to your program. See page 123 of the Stamp Manual.

Tracy Allen

Also, this statement

RANDOM flip
IF (flip >> 15) < 32767 THEN

should be changed to either

RANDOM flip
IF flip < 32768 THEN

or

RANDOM flip
IF (flip >> 15) = 0  THEN